Process for separating liquids from suspensions

ABSTRACT

Process for separating liquids from suspensions of ultrafine solids by moving a shearing device, such as a screen, through the suspension to concentrate the solids and permit the removal of the separated liquids.

BACKGROUND OF THE INVENTION

There are many commercial engineering operations as well as manylaboratory operations that produce suspensions of ultrafine solids in aliquid medium where the solid particles are very small and impart slowsettling characteristics to the suspension. Such systems may be termedcolloidal suspensions and frequently are called slimes in miningoperations. A typical example of the latter is the by-product producedfrom mining phosphates in Florida for use in producing fertilizers,phosphoric acid, or other phosphorous compositions. The phosphate occursnaturally with clays and after treatment to recover as much of thephosphate as possible, there is usually a byproduct slime containing 2-6percent solid material and the remainder water. The solids will notsettle in any reasonable length of time to permit recovery of the wateror of the solids which often contain significant amounts of valuableminerals. Many methods have been suggested for separating the liquidfrom the solids in such suspensions or slimes, among which areflocculation followed by precipitation or filtration, evaporation,centrifugation, freezing and thawing, and treatment withpolyelectrolytes followed by precipitation. While these procedures mayprovide some advantages in effecting a separation they each have certaindisadvantages which restrict the process to particular applications orwhich cause the process to be too expensive for general use.

BRIEF SUMMARY OF INVENTION

This invention relates to a process for accelerating the separation ofliquid from solids in a very slow-settling suspension or a slime, bymeans of a single mechanical operation which effects the compaction ofthe solids in a relatively short period of time. The process involvesthe movement of a relatively large mesh screen or net through thesuspension at a slow steady rate with the result that the solid materialis concentrated or compacted ahead of the moving screen leaving aclarified liquid behind the moving screen. In some instances, there maybe a flocculant material employed as an aid in agglomerating theultrafine solids, but this is not a step which is necesssary to theprocess of this invention. This process has been employed successfullywith phosphate slimes resulting from commercial mining operations inFlorida, and it has been found to be feasible to concentrate the solidmaterials to a much greater degree and in a much shorter period of timethan has been possible without the use of expensive machinery oradditives.

It is an object of this invention to provide a rapid method forseparating liquid from solids in very slow-settling suspension withoutthe necessity of employing chemical additives or using centrifuges orfilters.

Another object of this invention is to provide an improved process fortreating phosphate slime to recover clarified water and concentratedminerals therefrom.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an embodiment of this invention wherein a tank is filled witha very slow-settling suspension which is to be separated and a screen ismoved downwardly through the suspension in that tank in order to effectthe separation.

FIG. 2 shows an embodiment of this invention wherein a pond containing avery slow-settling suspension is clarified by dragging a screen from oneend of the pond to the other end.

FIG. 3 shows an embodiment of this invention wherein a pond filled witha very slow-settling suspension is clarified by means of a large areascreen which is moved slowly downward through the pond.

With specific reference to FIG. 1 a tank of any convenient size isrepresented at 1 and is filled with a very slow-settling suspension to alevel at 2. This suspension is treated by the process of this inventionby moving screen 3 downwardly through the depth of the liquid towhatever concentration is deemed to be advisable. The method shown foroperating the process of this invention in this embodiment involves acentral guide rod 7 and supporting structure 4 to which screen 3 isattached. Guide rollers 5 and 6 maintain the supporting structure 4 andscreen 3 in a position perpendicular to guide rod 7 as screen 3 movesdownwardly through the suspension. The means for actually moving thescreen 3 and supporting structure 4 is not shown in this sketch becauseany of a variety of alternatives may be employed. For example thesupporting structure 4 may be lowered at a desired rate by reason of itsown weight attached to cables and appropriate pulleys over the upperedge of tank 1 to a suitable winch or winches located outside the tank.Another alternative is to provide a driving arrangement of suitablegearing which connects guide rod 7 and /or tank 1 with moveablesupporting structure 4. Still other driving means are hydraulic pistonsconnected to the wall of tank 1 in a manner which forces supportingstructure 4 downward at any given rate of movement. The exact meansemployed for moving supporting structure 4 and screen 3 is completelyunimportant to the present invention since any suitable means may beemployed. As screen 3 moves downward it produces a concentration ofsolids at 8 immediately ahead of screen 3 and leaves a clarifiedsupernatant behind at 9, which can be pumped off (by means not shown) asthe process proceeds.

A desirable additional feature (not illustrated here) to be employed inthe embodiment of FIG. 1 is a rotating thickener rake in the bottom oftank 1. The rake is of conventional design with blades set at an angleso as to scrape the concentrated solids toward the center where they areremoved continuously. In this invention the blades are made of screenmaterial similar to that of screen 3 so that water may pass through theblade and solids may be concentrated by the movement of the blade. Thespeed of blade movement sould be the same relative linear speed as thatof screen 3 moving downward through the slow-settling suspension in tank1.

In FIG. 2 there is shown a cross-section of a pond filled with a veryslow-settling suspension to the level 20 with the walls and bottom ofthe pond at 21. In this embodiment the suspension is treated toconcentrate the solids therein by moving screen 23 from left to rightthrough the pond by the action of winch 26 reeling-in cable 25 at thedesired rate of speed to move screen 23 through the suspension in theapproved manner. Screen 23 is shown to be supported by float 24 fromwhich screen 23 hangs downwardly by gravity. Screen 23 could also besupported from above level 20 by a traverse mounted on rails mounted onrails running lengthwise of the pond. As the screen moves through thesuspension the solids are concentrated as shown at 27 leaving theclarified liquid at 28 and the unaffected dispersion at 29. It isobvious that this embodiment can take on several variations. For examplescreen 23 might not be a freehanging screen but could be supported froma rigid frame or alternatively the free-hanging screen could haveweights attached to the bottom portions to maintain it in a somewhatstable taut condition. One preferred embodiment is to employ 2, 3, or 4closely spaced screens aproximately parallel to each other in place ofthe single screen 23 in FIG. 2. This arrangement provides a an improvedclarification performance. Many other arrangements can be imagined bythose skilled in the techniques of engineering for accomplishing thissame result with variations in the procedure, and yet embodying theprinciple of this invention.

In FIG. 3 there is shown a pond similar to that described in FIG. 2wherein a suspension has been introduced up to a level 30 and the wallsand bottom of the pond are represented at 31. Over the surface 30 of thepond there is a platform 44 which may foat on the surface of the pond orbe rigidly attached to the walls or bottom of the pond (no method ofattachment being shown in this figure). On the platform is built asupport rigging 42 which supports a pulley 41 through which cable 40 isthreaded terminating in hook 39 to which is attached rod 38 hangingvertically downward through a hole in the platform 44. The other end ofcable 40 is attached to winch 43 which is driven by a means not shown ata desired speed. The lower end of rod 37 is fitted with a collar 35 anda suitable number of hinges 36 to which are attached struts 33. Theouter end of struts 33 is attached to screen 32 and the weight of struts33 causes screen 32 to be taut by the force of gravity. When the pond isfilled with suspension to the level 30 winch 43 is operated to raise theposition of screen 32 to coincide with the surface 30. In performing theseparation provided by this process screen 32 is slowly moved downwardlythrough the suspension at the desired rate to effect concentration ofthe solids ahead of the screen. In this drawing the solids are shownconcentrated at 45 while clarified liquid remains behind the screen at46. If a force greater than the weight of the assembly is needed to movethe screen 32 at the desired rate, a supplemental driving force can beprovided. In the operation of this invention there is a "drag effect"which causes concentration of solids to some extent outside theperimeter of screen 32. It is not known in full detail why this happensbut it has been observed that the solids being forced ahead by thescreen 32 are apparently attracted to other solids nearby and pull thosesolids along in the same direction to produce a considerable amount ofconcentration. For example when screen 32 has advanced far enough to beshown in the position 32A with the struts at 33A and collar at 35A theupper level of concentrated solids in the dispersions will beapproximately at the position shown at 47 with all of the liquid abovethat location being clarified and all of the substance below thatlocation comprising concentrated solids in liquid. Once theconcentration has reached that point the suction line of a pump can beplaced in the supernatant clarified liquid to pump off the liquid andleave the concentrated and compacted solids in the bottom for removal orfor further concentration, e.g. by evaporation.

There are many other embodiments which can be designed embodying theprinciple of this invention and it is not intended that this inventionshould be limited in any fashion by these drawings but only that theinvention be illustrated thereby.

DETAILED DESCRIPTION

This invention comprises a process for accelerating the separation ofliquid from solids in a very slow-settling suspension such as acolloidal suspension, by moving one screen or a series of screensthrough the suspension at a substantially steady linear rate of lessthan about 1 foot (30.5 cm.) per day whereby the solids are concentratedahead of the moving screen and the clarified liquid remains behind themoving screen. The suspension acts like a gel which can be compressed tosqueeze liquid out of the suspension so as to effect a separationbetween the liquid and the solids. This process is applicable not onlyto aqueous dispersions known as slimes which frequently result frommining operations, but it also applies to suspensions wherein the liquidis non-aqueous. The largest known application for this process is foundin the products resulting from mining operations wherein amineral-bearing clay is dug from the ground, valuable minerals arerecovered therefrom, and there is left as a by-product a stablesuspension (i.e. one which settles at an extremely slow rate) of solidmaterial in water. A typical example of such a by-product is thephosphate slime resulting from the treatment of phosphate clay mined inFlorida. Another typical product is the suspension resulting from thetreatment of bauxite to recover alumina and leaving a red clay slimewhich can be separated only with difficulty to recover the water andmore alumina.

In the process of this invention a screen or netting, preferably of arigid or semi-rigid construction, is moved slowly but steadily throughthe suspension at a rate which does not exceed about 1 foot (30.5 cm.)per day with the result that the solids are concentrated ahead of themoving screen leaving clarified water behind.

The screen which is employed can be a very large mesh screen generallyvarying from about 2-mesh to about 35-mesh in size. Smaller size screenswill operate successfully but they provide a greater resistance tomovement and thus it is advantageous to provide the largest openingsthat are feasible. The screen is desirably made of metal so as toprovide some stiffness and rigidity, but it is entirely feasible toemploy fiber or cloth netting, which for example might be drawn tautacross a frame and produce the same results.

In certain embodiments of this invention it may be advantageous toemploy multiple pairs of screens to simultaneously concentrate solids atseveral locations in the same suspension. For example, in the embodimentof FIG. 1, there may be two or more parallel screens 3 simultaneouslymoving downwardly through the suspension, each moving toward a separatefixed screen (substantially the same as screen 3 although preferably ofslightly smaller mesh than the moving screen). Thus with a fixed screenat every 2 feet (61 cm.) of depth and a movable screen cooperating witheach fixed screen and travelling toward the fixed screen at about 1 foot(30.5 cm.) per day, the entire tank of suspension can be clarified intwo day's time. At each fixed screen there will be a concentration ofsolids produced by the motion of a travelling screen toward itsrespective fixed screen. This produces several layers of clarifiedliquid which can be drained off at one or at several levels if thedrawoff rate is sufficiently low to prevent disturbance of the layers ofconcentrated solids.

It is somewhat difficult to predict precisely the optimum rate ofmovement of the screen through a given suspension to produce a desiredseparation. Nevertheless, it has been found that a maximum rate of about1 foot per day is a practical limit. In operations on phosphate slimesit has been found that rates of about 4 to about 12 inches (10.2 to 30.5cm.) per day represented optimum speeds for producing the desiredseparation.

The concentration effect of moving the screen through the suspension isnot limited to the exact area of the screen. It appears that surroundingportions of the suspension are also compacted somewhat, even though theyare not physically in contact with the screen. It is therefore anembodiment of this invention to employ a multiplicity of screens whichare not connected with one another but are spaced apart from each otherover the horizontal area of the surface as they move through thesuspension. There is a "drag effect" which simultaneously causes someconcentration of the solids beyond the perimeter of a screen although toa lesser degree than the concentration of solids immediately ahead ofthe screen.

It is contemplated that in the process of this invention there may beinstances where it is advantageous to employ a flocculant material tocause the particles in the solid suspension to agglomerate and therebyto facilitate concentration. Such a flocculant material may be added tothe entire surface of the suspension. In a particular embodiment of thisinvention it has been found that if the flocculant is added continuouslyor periodically at the interface between the moving screen and theconcentrated solids a desirable effect is produced with a greatlyreduced amount of flocculant because the flocculant dosage is based onthe relatively small volume of suspension available to the flocculant atthe interface of the unclarified suspension and the supernatant. Theaddition of flocculant to this interface can be readily accomplished byincorporating into the supporting structure for the screen a means forinjecting flocculant as desired. The supporting structure may, forexample, include hollow conduits for distributing the flocculant to anydesired location or, alternatively, the supporting structure may alsosupport a separate feed-system of tubing for distributing the flocculantat the desired locations. Among the flocculant materials which may beemployed are starches such as those obtained from corn, potato,arrowroot, etc.; and polyelectrolytes, which may be anionic or cationic.Typical of the polyelectrolytes are polyacrylamide or its salts,hydrolyzed polyacrylamide or its salts, polyacrylic acid or its salts,etc. Many other materials are known to be useful as flocculants, and itis not critical to this invention that any particular variety beemployed. It is only important that it have the ability to agglomeratethe colloidal particles so as to facilitate the concentration of thesolids.

There are many methods for employing this process in a commercialprocess. Since it is common practice for an industry to collect a veryslow-settling suspension or slime in a large open pond, methods must beavailable for concentrating these solids in such ponds. One of suchmethods is to employ a barge or raft which floats on the surface of thepond and from which a large screen is supported and is pushed downwardthrough the liquid at the required rate of travel. The screen might be asingle unit or there can be several separate units working in concertand located closely adjacent to each other over the surface area of thepond. As a single unit the raft could be fitted with a rod capable ofsupporting the large screen in a position parallel to the surface of thepond and with means of moving the rod and screen downward at the slowrate of travel indicated previously herein. It is not necessary that thescreen have an overall area comparable to the entire area of the pondbecause concentration occurs beyond the perimeter of the screen througha "drag effect." Accordingly, a barge or raft located in the center ofthe pond can concentrate the solid material directly below the screen toa maximum extent while concentration occurs to a lesser extent furtherand further away from the screen with the result that the volume ofclarified liquid assumes a shape approaching that of an inverted cone.Alternatively, there might be several independent crane structures on asingle raft each supporting a separate screen and means forindependently advancing the screen downward through the suspension atthe required rate. If the screens are relatively close to each other,the solids will concentrate almost equally well between the screens asimmediately ahead of each screen. In this fashion a relatively largearea can be concentrated at one time.

Another procedure for employing the process of this invention in acommercial fashion is to provide enough large tanks to handle theeffluent suspension from the industrial operation and to provide eachtank with a large screen and a means of moving the screen downwardlythrough the depth of the tank at the desired rate of travel. Byutilizing this type of operation the first tank to be filled isimmediately subjected to the concentration operation, which might, forexample, require 3-6 days. While the first tank is being subjected tothe concentration process one or more other tanks are filled and theconcentration treatment is started. After sufficient concentration thesupernatant liquid is pumped off, the solids removed, and the operationis started again. Another procedure which may be used in an open pond isto employ a net which stretches over a large area of the pond and whichhas a mesh size of the specifications mentioned above. The net isweighted so that it will move slowly down through the liquid at thedesired rate or it can be driven downwardly by means described above. Itis also possible to perform the concentration in a horizontal directionrather than in a vertical direction and for this purpose a screen or netmight be supported across the width of the pond and the entire netdragged the length of the pond at the linear rates mentioned above toproduce the concentration of solids ahead of the screen and to leave theclarified liquid behind the screen. This invention also encompasses theembodiment wherein the screen remains stationary and the suspension ismoved through the screen at a very slow rate such that the relativemovement of the screen to the suspension is within the limits of lineartravel given above. A means for employing this concept in a commerciallyacceptable fashion is to place in the pond a large diameter pipe orseveral small diameter pipes with pipe walls constructed of the screenhaving the mesh size described above. The liquid is pumped from the pipeat such a rate that the suspension outside the pipe moves toward andthrough the pipe wall at the linear rates given above. This issubstantially the reverse of the embodiments mentioned above but it isan operable procedure which may be preferred in certain situations.

It has been found advantageous in certain embodiments of this inventionto provide a forwardly projecting shearing element or a multiplicity ofsuch elements to pierce and shear the compacting solids immediatelyahead of the moving screen. These shearing elements can take any ofseveral forms and shapes but a common one is the shape of a pin orneedle projecting forward ahead of the screen into the aera of thesolids that are being concentrated. It is of course not necessary thatthis be rod-like since the shearing element could be in the form of aknife blade or other type of projection which does not impede the flowof liquid through the screen nor the concentration of the solids aheadof the screen. The actual size of the projections will vary with thesize of the screen and the dimensions of the concentrated solids.Normally the length of the projections will not be more than 1-2 feet ina commercial operation. It is advantageous to employ a large number ofsuch projections in a single screen with each projection being small indiameter, e.g. not exceeding one inch. The precise reason why suchprojections are useful is not fully known but it is believed that it hassomething to do with providing easy exits for water to flow away fromthe concentrating solids and thus facilitates the concentrating action.These projections are not necessary to the operation of this process,they are merely expedients to be used in certain situations where itwould appear to be of assistance in operating the process.

Another embodiment of this invention is a slowly rotating thickener withrakes constructed of screen material at the bottom of the containerwhich holds the suspension. The thickener rake elements are fabricatedof screen to obtain improved performance over thickeners withconventional rakes by concentrating the solids as described above whenthe screen shears the suspension as it moves through the liquid. Thisembodiment is operable to effect concentration when used by itself, oralternatively it can be used in conjunction with other moving screens asdescribed in the foregoing explanation of this invention.

Although this invention has been described in detail with reference tocertain embodiments thereof, it will be understood that variations andmodifications can be effected within the spirit and scope of theinvention and as defined in the appended claims.

I claim:
 1. A process for accelerating the separation of water fromsolids contained in phosphate slimes which comprises moving a porousshearing device having openings smaller than 2 mesh but substantiallylarger than the particles contained in said slimes through said slimesat a substantially linear rate of less than one foot per day whereby thesolids are concentrated ahead of the moving shearing device.
 2. Theprocess of claim 1 wherein said shearing device is a screen.
 3. Theprocess of claim 2 wherein said screen is from 2-mesh to 35-mesh insize.
 4. The process of claim 1 wherein a flocculant material is addedto the suspension.
 5. The process of claim 4 wherein the flocculantmaterial is added to the suspension at the leading surface of theshearing device.
 6. The process of claim 1 wherein said rate is fromabout 4 to 12 inches per day.
 7. The process of claim 2 wherein saidscreen is a rigid to semi-rigid structure.